The present invention relates generally to a method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to a method for eliminating any ringing, such as magnetoacoustic ringing, during a nuclear magnetic resonance measurement.
Borehole nuclear magnetic resonance measurements provide different types of information about a reservoir. First, the measurements provide an indication of the amount of fluid in the formation. Second, the measurements present details about whether the fluid is bound by the formation rock or unbound and freely producible. Finally, the measurements can be used to identify the type of fluid--water, gas, or oil.
One approach to obtaining nuclear magnetic resonance measurements employs a locally generated static magnetic field, B.sub.0, which may be produced by one or more permanent or electromagnets, and an oscillating magnetic field, B.sub.1, which may be produced by one or more RF antennas, to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, and permeability of a formation. See U.S. Pat. Nos. 4,717,878 issued to Taicher et al. and 5,055,787 issued to Kleinberg et al. Nuclear spins align with the applied field B.sub.0 with a time constant of T.sub.1 generating a nuclear magnetic moment. The angle between the nuclear magnetization and the applied field can be changed by applying an RF field, B.sub.1, perpendicular to the static field B.sub.0. The frequency of the RF field is equal to the Larmor frequency given by .omega..sub.0 =.gamma.B.sub.0 where .gamma. is the gyromagnetic ratio. After application of an RF pulse, the magnetization begins to precess around B.sub.0 and produces a detectable signal in the antenna. The signal detected by the antenna includes a parasitic, spurious ringing that interferes with the measurement of spin-echoes.
The source of the spurious signal is electromagnetic generation of ultrasonic standing waves in metal. See E. Fukushima and S. B. W. Roeder, Spurious Ringing in Pulse NMR, 33 J. MAGN. RES. 199-203 (1979). As explained in the Fukushima et al. article, the induced RF current within the skin depth of the metal interacts with the lattice in a static magnetic field through the Lorenz force and the coherent ultrasonic wave propagates into the metal to set up a standing wave. A reciprocal mechanism converts the acoustic energy, in the presence of the static field, to an oscillating magnetic field which is picked up by the antenna as a spurious, ringing signal.
Different types of magnetoacoustic interaction may produce a parasitic signal in the NMR antenna. Antenna wiring and other metal parts of the NMR logging tool can be affected by the static magnetic field and the RF field generated by the antenna. If the antenna is located within the strongest part of the magnet's field, when RF pulses are applied to the antenna, acoustic waves are generated in the antenna and the antenna sustains a series of damped mechanical oscillations in a process known to those skilled in the art as magnetoacoustic ringing. This ringing can induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
Another source of magnetoacoustic interaction is magnetorestrictive ringing which is typically caused when non-conductive magnetic materials, such as magnetic ferrite, are used in the antenna. If this magnetic material is located within the strong part of the RF field, application of RF pulses will generate acoustic waves in the magnet. The magnet will experience a series of damped mechanical oscillations upon cessation of the RF pulse. Magnetorestrictive ringing can also induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
One type of NMR well logging apparatus which reduces magnetoacoustic interaction is described, for example, in U.S. Pat. No. 5,712,566 issued to Taicher et al. The apparatus disclosed in the '566 patent includes a permanent magnet composed of a hard, ferrite magnet material that is formed into an annular cylinder having a circular hole parallel to the longitudinal axis of the apparatus. One or more receiver coils are arranged about the exterior surface of the magnet. An RF transmitting coil is located in the magnet hole where the static magnetic field is zero. The transmitting coil windings are formed around a soft ferrite rod. Thus, magnetoacoustic coil ringing is reduced by the configuration of the transmitting coil. Magnetorestrictive ringing of the magnet is reduced because the radial dependence of the RF field strength is relatively small due to use of the longitudinal dipole antenna with the ferrite rod. Further, magnetorestrictive ringing is reduced because the receiver coil substantially removes coupling of the receiver coil with parasitic magnetic flux due to the inverse effect of magnetorestriction.
The apparatus disclosed in the '566 patent has several shortcomings. First, the permanent magnet material must be electrically nonconductive so that the antenna used to generate a radio frequency magnetic field can be located in the hole. Second, by placing the antenna in the hole, the efficiency of the antenna is decreased due to the distance from the antenna to the formation. The '566 patent describes an alternative embodiment wherein the magnet hole is radially displaced towards the outer surface of the magnet. In the preferred and alternative embodiments of the '566 patent, locating the antenna in the magnet hole increases the radial distance from the antenna to the volume of investigation in the formation. In cases of substantial borehole rugosity, the volume of investigation may be positioned within the borehole itself rather than wholly within the earth formation.
Normally, magnetoacoustic interaction caused by a 180.degree. pulse of a CPMG sequence is eliminated by a phase alternating pulse sequence. As described, for example, in U.S. Pat. No. 5,596,274 issued to Abdurrahman Sezginer and U.S. Pat. No. 5,023,551 issued to Kleinberg et al., a pulse sequence, such as the Carr-Purcell-Meiboom-Gill (CPMG) sequence, first applies an excitation pulse, a 90.degree. pulse, which causes the spins to start precessing. After the spins are tipped by 90.degree. and start to dephase, the carrier of the refocusing pulses, the 180.degree. pulses, is phase shifted relative to the carrier of the 90.degree. pulse according to the sequence: EQU CPMG(.sup..+-.)=90.degree..sub..+-.x [t.sub.cp 180.degree..sub.y t.sub.cp .+-.echo.sub.j ],
where the bracketed expression is repeated for j=1, 2, . . . J, where J is the number of echoes collected in a single CPMG sequence, and t.sub.cp is half of the echo spacing. 90.degree..sub..+-.x denotes an RF pulse that causes the spins to rotate by a 90.degree. angle about the .+-.x-axis (phase alternated). Similarly, 180.degree..sub.y denotes an RF pulse that causes a 180.degree. rotation about the y-axis. The ringing due to the 180.degree. pulse is eliminated by combining a pair of phase alternated CPMG sequences, that is, subtracting the echoes in CPMG(.sup.-) from the echoes in the neighboring CPMG (.sup.+). Generally, the ringing due to the 90.degree. pulse is ignored. In addition to ringing, the electronic measuring circuit may introduce a baseline shift which makes the measurement of the absolute echo intensity more difficult. The phase alternated pulse sequence operation also cancels the spurious baseline that may be present in the measurements.
A drawback to the phase alternated sequence is the requirement to measure two pulse sequence cycles. Measurements made by an NMR logging tool in this manner are therefore subjected to degradation in the vertical resolution due to the logging speed, wait time between each pulse sequence, and the data acquisition time. In addition, the logging tool moves along the longitudinal axis of the borehole between each of the measurements. Possibly, the echoes from the CPMG(.sup..+-.) sequences are measured with the tool facing different formations wherein each formation has a different conductivity. Laboratory tests show that magnetoacoustic interaction is affected by the formation conductivity.
FIGS. 1a-1c present the experimental results of an NMR measurement where the phase alternated pulse sequences are measured at two different conductivities. The positive phase cycle measurement (FIG. 1a) is obtained from a 0.25 .OMEGA.-m water sample and the negative phase cycle measurement (FIG. 1b) is obtained from a 0.9 .OMEGA.-m water sample. FIG. 1a shows the positive phase echoes, baseline offset, and minimum ringing from the 180.degree. pulse while FIG. 1b illustrates the negative phase echoes, baseline offset, and substantial ringing from the 180.degree. pulse. When these signals are combined by subtracting the signal obtained during the negative phase cycle from the signal obtained during the positive phase cycle, the result depicted in FIG. 1c is obtained which shows that the ringing and baseline offset are not completely canceled with the phase alternated pulse sequences.